定义：当波长大于某一值时，某特定模式不再存在，该波长就称为此模式的截止波长。

波导（如光纤）中传播的模式的数量取决于光的波长：波长越短，可以传播的模式越多。对于长波长的光，其传输模式很可能只有一个模式（参阅单模光纤），也有可能没有可以传输的模式；但是对于短波长的光，其传输模式很有可能有很多个（参阅多模光纤）。 

当一个特定的模式在高于某一波长时不再存在，该波长就是其截止波长。对于光纤，LP11模式的截止波长会限制其单模区间，因为低于该波长至少会有LP01和LP11模式。 

当低于截止波长时，各个模式的性质会有很大的差异。通常模式半径（以及有效模式面积）在截止波长附近会显著变大，因此在纤芯中传播的能量也就大幅变小。图1显示了在多模渐变折射率光纤中的此效应，在其它的折射率分布的光纤中也有相似的性质。 

图 1： 不同模式的在光纤纤芯中的能量比例随波长变化。纵向的直线给出了不同模式的截止波长。 

对于光纤中的LPlm模式，只有当l=0时，当波长接近截止波长时，处于纤芯中的能量分量才变成0（即模场面积变成无限）。对于l大于0的情况，模场面积依旧为有限值。

在阶跃折射率光纤中，基模（LP10）没有截止波长，但是对于其他结构的光纤（尤其是光子晶体光纤中），基模也有对应的截止波长。 

对于没有径向对称性的光纤（和强烈弯曲的光纤），截止波长可能会与偏振相关。

Definition: a wavelength above which a guided mode of a waveguide ceases to exist

Formula symbol: λ_co

Units: m

The number of guided modes of a waveguide (for example, an optical fiber) depends on the optical wavelength: the shorter the wavelength, the more modes can be guided. For long wavelengths, there may be only a single guided mode (→ single-mode fibers) or even none at all, whereas multimode behavior is obtained at shorter wavelengths.

When a particular mode ceases to exist beyond a certain wavelength, that wavelength is called its cut-off wavelength. For an optical fiber, the cut-off wavelength for the LP₁₁ mode sets a limit to the single-mode regime, as below that wavelength there is at least the LP₀₁ and the LP₁₁ mode.

Just below the cut-off wavelength, the mode properties often vary substantially. Typically, the mode radius (and thus the effective mode area) increases sharply near the cut-off, and the fraction of power propagating within the waveguide core decreases accordingly. That effect is shown in Figure 1 for a multimode step-index fiber; similar behavior occurs for fibers with other transverse refractive index profiles.

Figure 1: Fraction of the power of various guided modes (where the colors are related to the l indices of those) which is contained in the fiber core as a function of the wavelength. The thin vertical lines indicate the calculated cut-off wavelengths of the modes. The diagram has been produced with the software RP Fiber Power.

For LP_lm modes of a fiber, only for l = 0 the fraction of the power guided in the core goes to zero when approaching the cut-off. For modes with higher l, the mode size stays finite there.

In step-index fibers, there is no cut-off for the fundamental (LP₀₁) mode. For other fiber designs, in particular for some photonic crystal fibers, there can also be a fundamental cut-off.

Fibers with not radially symmetric designs (and strongly bent fibers) can have polarization-dependent cut-off wavelengths.

Just below its cut-off wavelength, the bend losses of a mode can become very high due to the increased mode area. Therefore, even for moderate bending of the fiber one may obtain sharply increasing propagation losses near the cut-off wavelength. Therefore, cut-off wavelengths can not always be precisely determined in experiments.